A bipolar junction transistor (BJT) is a three-terminal device having two P-N junctions between n-type and p-type semiconductor materials that can be arranged in a PNP and NPN configuration. The three terminals are often referred to as an emitter, a base, and a collector.
High power bipolar transistors can have multiple emitters to achieve high currents, and to provide efficient use of the whole emitter area. These emitters experience high current densities and are self-heated above ambient temperatures, leading to concerns about thermal run-away and damage to the device.
Large area bipolar transistors can also have a very non-uniform current distribution due to the resistance of the base layer. Since the base current is applied through a thin base layer, there can be a significant series resistance in large devices. This resistance causes a voltage variation across the base region. This voltage variation then causes a variation of the emitter current density, especially since the emitter current density depends exponentially on the local base-emitter voltage. This effect is minimal in the center of the emitter-base junction and strongly increases toward the edges. In extreme cases, this effect causes the emitter current to occur only at the very edges of the emitter-base junction. The parameters involved include the sheet resistance of the base layer, the emitter current density and the current gain in the device.
Therefore, improved multiple-emitter BJT designs with reduced resistance, and techniques for fabrication thereof, would be desirable.